1. Field of Invention
This invention pertains generally to the field of multicrystalline silicon solar cells and improvements to their manufacture directed to improved electrical and optical performance. More specifically, the invention pertains to tailoring emitter silicon doping profiles and enhancing light absorption of solar cells using an engineered texturing treatment compatible with standard solar cell manufacturing processes.
2. Description of the Related Art
Multicrystalline silicon (mc-Si) is a frequently used semiconductor substrate in the manufacture of silicon solar cells. Single crystal silicon (sc-Si), while exhibiting favorable electrical characteristics related to the presence of fewer crystal imperfections as compared with mc-Si is much more energy and cost intensive to produce than mc-Si, and therefore, the commercial market in silicon solar cells depends significantly on mc-Si.
Various phenomena effect the cell's electrical efficiency and response to different wavelengths of light. Optimal solar cell performance depends on at least three factors: maximized absorption of light which causes generation of free electrical charges within the semiconductor matrix of the cell due to the photoelectric effect; minimized recombination, which is annihilation of charges within the cell before they enter electrical contacts affixed to the semiconductor matrix; and minimized contact resistance at the junction between the crystalline semiconductor portion of the cell and the metal contacts used to collect charges and route current outside of the cell.
Maximizing light absorption may involve application of anti-reflective coatings on solar cell surfaces exposed to light. Another technique is to create rough or textured surfaces on cells to increase the amount of incident light that ultimately enters the semiconductor matrix of the cell. Oftentimes, the combination of both of these approaches is useful.
As noted, recombination causes a decrease in the current-generating capacity of a cell. Examples of factors that can increase recombination include presence in the semiconductor matrix of recombination centers (traps) where holes and electrons recombine and negate the benefits associated with the photoelectric effect, and crystal impurities that result in the presence of regions or surfaces with dangling bonds capable of binding charge carriers. Techniques for minimizing recombination include gettering and passivating surface and bulk portions of the semiconductor material using atomic hydrogen. The effect of each of these techniques is to remove or neutralize crystal imperfections that interfere with the movement of free charge carriers.
Minimizing contact resistance can be less straightforward because of the optical and electrical characteristics of silicon semiconductor material. A low-cost method for positioning metal contacts on mc-Si solar cell surfaces is by screen-printing gridlines. The metal contacts are needed to collect and utilize the free electrons generated by the photoelectric effect. Often, the metal contacts contain silver applied in the form of particles suspended in an organic paste. During processing the organic component in the paste is burned off, leaving the silver affixed to the surface of the cell in a desired configuration. Given this context, the standard approach in industry for reducing contact resistance is to dope the cell emitter using phosphorus in the region where the metal contacts are to be positioned. Although this technique improves the electrical connection between the metal gridlines and the surface of the cell, the presence of P atoms within the crystal matrix can cause an increase in recombination losses, for example, where the P atoms are infused near the cell surface, known as the emitter. Recombination in this upper region of the cell, where light in the blue range of the spectrum is most likely to be absorbed, causes the cell to exhibit a poor blue response. Common techniques for minimizing this detrimental effect include either lightly doping the cell surface or partially etching the cell surface after doping to reduce the amount of P in the crystal. While this preserves some blue response in the cell, it is counterproductive as regards reducing the contact resistance where the P concentration is low in the region of the junction between the cell surface and the metal contacts.
Efforts have been made to selectively engineer the doping profile of solar cells so as to derive maximum benefit from P doping in the region of contact metallization while minimizing doping between contacts. This selectively pattered emitter doping profile (selective emitter) has historically been obtained by using expensive photolithographic or screen-printed alignment techniques and multiple high-temperature diffusion steps. J. Horzel, et al., Proc. 26.sup.th PVSC, September 1997, pp.139-142. Another technique used to improve cell performance is plasma-enhanced chemical vapor deposition (PECVD). This is recognized as a potentially cost-effective way to provide simultaneous surface passivation and an effective anti-reflection coating. To gain the full benefit from improved emitter-surface passivation on cell performance, though, it is still desirable to tailor the emitter doping profile so that the emitter is lightly doped between the gridlines, but heavily doped beneath them. This is especially true for screen-printed gridlines, which require very heavy doping beneath the grid for acceptably low contact resistance.
The need exists, therefore, for cost and energy efficient technology that permits selectively doping the emitter profile of mc-Si solar cells which is compatible with commonly employed methods of solar cell manufacture. Additionally, there is a need for texturing methodologies that are likewise compatible with existing manufacturing techniques.